Could be alot things, the first thing to determine is what kind of multi-meter? Is it auto ranging? The reason I ask is you should try and find out what sort of input into the multi-meter causes a fault.

As far as the amp, I would not use it until you figure out the problem. Check to make sure the tubes are all seated properly. It could be a faulty grid cap. Like I said try to figure out what input into the multi-meter causes a falut.